This invention relates to servo control apparatus and, more particularly, to such apparatus which finds ready application in a video signal recording/reproducing system, whereby the driven member, which may be a rotary transducer or the movable record medium, has its speed and phase controlled in synchronism with the horizontal and vertical synchronizing signals included in the video signal.
In a video signal recording/reproducing system, such as a video tape recorder (VTR), a transducer, such as a rotary magnetic head, scans the record medium, such as magnetic tape, as the medium is moved. In the VTR, video signals are recorded on the head in parallel, skewed record tracks; and these video signals subsequently are reproduced to result in a corresponding video picture. For improved quality in a VTR, either for use in television broadcast applications or for home entertainment, the rotation of the magnetic head and/or the movement of the tape should be synchronized with external synchronizing signals. That is, and with respect to the rotary head, for example, the rotational speed of the head, as well as its phase, or position, should be synchronized. With respect to the tape, which generally is driven by a capstan, the speed and position of that tape likewise should by synchronized.
In one type of synchronizing control apparatus, known as servo control apparatus, for controlling the rotary speed and position of the magnetic head, the speed is controlled by one servo loop and the position is controlled by another. For example, a frequency generator, such as a magnetic-toothed wheel and a magnetic pick-up, is provided on or driven with the shaft which drives the rotary magnetic head. The frequency of the signal produced by this frequency generator is a function of the rotary speed of the head. This frequency is compared with a reference frequency, such as the frequency of a signal derived from the horizontal synchronizing signal included in a video signal, and any difference therebetween is used to adjust the speed at which the head is driven. In the position-control servo loop, a position pulse generator, such as a magnetic element secured to the rotary head drive-shaft and a magnetic pick-up, generates a position pulse each time that the head rotates into a predetermined position with respect to the tape, such as into initial magnetic contact with the tape. The position pulse is phase-compared to the vertical synchronizing signal included in the video signal; and any phase difference therebetween is used to briefly change the speed of the head so as to adjust its phase. The two servo loops cooperate first by adjusting the phase of the rotary head to within a predetermined desired range, and then to control the rotary speed while maintaining the phase within this desired range.
However, since the speed and phase control of the aforementioned servo control apparatus requires the use of two separate and independent servo loops, this apparatus is of relatively complex construction. Furthermore, if the gain of the phase control servo loop is large so as to limit the acceptable range of desired position, hunting often occurs whereby the phase of the rotary head overshoots and undershoots, i.e., hunts, about its desired, limited range. Conversely, if the gain of the phase control servo loop is reduced so as to avoid such undesired hunting, it may require a long time delay until proper phasing, or positioning, of the head is attained. These factors must be taken into account in designing servo control apparatus, thus making the design of such apparatus quite difficult.